Not sure that 3 - 2" interconnect pipes will carry much heat with a chamber that big and heavy. It will likely be be very inefficient for fuel use as well, due to the bulk of the heat being held in the fire box and not transmitted effectively to the smoke chamber. Also, bottom mounted fire box translates into problems with uneven cooking throughout the smoke chamber, even with baffle/tuning plates. The need for baffling/tuning further reduces efficiency..been there with a SnP 40"...it was never-ending process of evaluating grate temps with nearly a dozen oven rack therms.
Another major factor is vent stack location(s), height above grate, etc. With a centrally located firebox, venting will be far more tricky in order to draw the heat accross the grate evenly. Too much draw from the wrong area will likely create a hot spot near and around the vent's inlet, as this is where the most air is moving to, and higher temps generally will result. Areas farther away from the vent will suffer somewhat stagnent air flow and cooler temps.
Just saying, it's difficult to get a good balance of heat distribution to the smoke chamber along with even dispersion of ventilation with this design type.
Have you considered a reverse flow side fire box? Very good performance overall with that design, and the best for horizontals...